Predator–prey interactions between phytoplankton and zooplankton represent a long-term evolutionary arms race in marine ecosystems. Toxic Pseudo-nitzschia species can respond to grazing pressure through multiple anti-grazing defenses, including morphological changes, enhanced silicification, and increased production of domoic acid (DA). However, how ocean warming modulates these defense–counter defense interactions and their ecological consequences remains poorly understood.
In this study, we investigated the combined effects of temperature and copepod grazing on two DA-producing Pseudo-nitzschia species using the copepod Pseudodiaptomus annandalei. Co-culture experiments were conducted at 19, 22, and 25 °C to evaluate algal physical defenses, chemical defenses, metabolomic responses, and the response of copepod including feeding and survival. Grazing pressure significantly induced multiple defensive traits in Pseudo-nitzschia. Both species exhibited increased cell volume, reduced surface-area-to-volume ratio, and enhanced biogenic silica content, indicating coordinated morphological and structural defenses. Grazing also promoted intracellular DA accumulation and increased dissolved DA concentrations, suggesting elevated toxin burdens in the surrounding environment. Temperature regulated these responses nonlinearly: lower temperature enhanced inducibility, whereas higher temperature increased absolute toxin levels and amplified ecological risk. Metabolomic analyses further revealed that predator pressure shifted algal metabolism from growth-oriented pathways, such as amino acid and nucleotide metabolism, toward defense-related pathways, including lipid metabolism, secondary metabolism, membrane-associated processes, and alkaloid biosynthesis. These results suggest that DA accumulation, silicification, and morphological changes are not isolated traits but integrated components of a systemic metabolic reprogramming from growth to defense.
For copepods, warming did not substantially alter grazing rate but increased mortality and intensified the adverse effects of toxic prey exposure. Overall, our results demonstrate that warming and grazing pressure interact to strengthen the defensive capacity of toxic Pseudo-nitzschia, weaken copepod top-down control, and promote shifts toward more toxic and highly silicified algal populations. These findings provide mechanistic insight into how ocean warming may enhance the intensity and ecotoxicological risks of harmful Pseudo-nitzschia blooms.